The invention relates to methods for making high wet performance webs.
Webs having a high strength when they become wet (known in the art as wet strength) are useful for many applications. One application for such webs is as premoistened tissues, often used by travelers for cleansing the body. Such webs or tissues must maintain sufficient strength when stored in wet conditions for an extended period of time to withstand wiping and rubbing actions. Other applications for high wet strength webs is in articles that need to maintain integrity when wetted with body fluids, such as urine, blood, mucus, menses and other body exudates.
In the art of papermaking, chemical materials exist for improving the wet strength of paper. These materials are known in the art as xe2x80x9cwet strength agentsxe2x80x9d and are commercially available from a wide variety of sources. For example, a polyamide/ polyamine/ epichlorohydrin resin is often used to enhance the wet strength of paper. This cationic resin is typically added to the papermaking slurry whereupon it bonds to the anionically charged cellulose. During the papermaking process the resin crosslinks and eventually becomes insoluble in water. The agent thus acts as a xe2x80x9cgluexe2x80x9d to hold the paper fibers together and enhances the wet strength of the paper. However, one needs to use chlorine in order to remove the resin and recycle products containing this resin, which presents environmental problems.
Cationic resins have other disadvantages, such as reacting with other anionic additives which it may be advantageous to add to the paper and, in many cases, increasing the dry strength of the paper as well, resulting in a less soft paper. Moreover, the effectiveness of cationic wet strength agents can be limited by low retention of the agent on the cellulose fiber.
The use of formaldehyde and various formaldehyde addition products to crosslink cellulosic fibers is known in the art. However, formaldehyde is an irritant and a known carcinogen. Crosslinking with compounds comprising formaldehyde at elevated temperatures can be particularly rapid relative to many other crosslinkers, requiring times as low as 1 to 10 seconds. However, for higher molecular weight compounds and for formaldehyde-free crosslinkers in general, much longer reaction times are found.
Other references disclose absorbent structures containing individualized, crosslinked fibers, wherein the crosslinking agent is selected from the group consisting of C2 to C8 dialdehydes, with glutaraldehyde being desired. The cost associated with producing fibers crosslinked with dialdehyde crosslinking agents such as glutaraldehyde may be too high to result in significant commercial success.
The use of monomeric polycarboxylic acids to impart wrinkle resistance to cotton fabrics is known. A cellulosic material was impregnated with a solution of the polycarboxylic acid and a catalyst, followed by drying the material and then curing the material in an oven at 150xc2x0 C. to 240xc2x0 C. for 5 seconds to 30 minutes.
The prior art also teaches a method of imparting wrinkle resistance to cellulosic textiles by crosslinking monomeric cyclic aliphatic hydrocarbons having multi carboxylic acid groups to the cellulose. Curing is said to be performed at about 150xc2x0 C. to 240xc2x0 C. for 5 seconds to 30 minutes.
The use of C2 to C9 monomeric polycarboxylic acids to make individualized, crosslinked cellulosic fibers having primarily intra-fiber crosslinking (crosslinks between cellulose units in a single fiber) and purportedly having increased absorbency has been taught.
Polyacrylic acid has been taught as a crosslinking agent, preferably as a copolymer with polymaleic acid. The fibers were fiberized prior to curing to make individualized, crosslinked cellulosic fibers having primarily intra-fiber crosslinking. The fibers are purportedly useful in absorbents. The crosslinking was achieved using temperatures of about 120xc2x0 C. to 160xc2x0 C.
Various resinous maleic anhydride compositions have been used in conjunction with paper products. For example, prior art discloses paper products coated with a composition including an amine salt of a low molecular weight C6 to C24 olefin/maleic anhydride copolymer in combination with a bisulfite. Such paper products exhibit release properties. Various amine salts of half esters of maleic anhydride/alpha-olefin copolymers have been disclosed as useful paper sizing or water holdout agents. Similarly, prior art discloses paper products impregnated with a sizing and wet strength agent of a reaction product of an alkyl tertiary amino alcohol and a copolymer of maleic anhydride/styrene or derivatives thereof. The use of an agent consisting of epoxide resins and maleic anhydride copolymers as an agent for imparting wet strength is known.
Polymeric treatment agents for adding wet strength to paper, which can be applied to a slurry or to a paper web, wherein curing times are said to range from 5 minutes to 3 hours, with a desired time range of 10 to 60 minutes, have been disclosed. The application of a polymeric polyacid, a phosphorous containing accelerator, and an active hydrogen compound to a paper web followed by curing at 120xc2x0 C. to 400xc2x0 C. for 3 seconds to 15 minutes has also been disclosed.
Accordingly, what is needed is a method of improving the wet performance of cellulosic based webs using non-formaldehyde crosslinking agents.
The present invention is directed to methods for improving the wet performance of cellulosic webs. The methods impart high wet resiliency, high wet strength, and a high wet/dry strength ratio to wet-formed webs. The methods include applying a polymeric anionic reactive compound (PARC) solution onto a web, with subsequent curing.
In one embodiment of this invention, the PARC is applied heterogeneously to the web, with heterogeneity due to the z-direction distribution of PARC or due to the distribution of the PARC in the plane of the web. Thus, in one embodiment, the PARC may be applied in a particular pattern such as a series of lines or sinusoidal waves extending in a first direction such as the machine direction to provide high wet performance in that first direction by virtue of continuously extending treated zones.
Heterogeneous application of polymeric anionic reactive compound can produce sheets that have regions of high wet strength or high wet resiliency, where polymeric anionic reactive compound has been applied, separated by regions of relatively lower stiffness where polymeric anionic reactive compound has not been applied. Thus, a web can have wet resiliency or wet strength properties and flexibility at levels that cannot readily be obtained in a uniformly treated web.
Heterogeneous application of the PARC to the web can be achieved in several ways, such as by gravure printing, flexographic printing, offset printing, and application through a mask or stencil.
The polymeric anionic reactive compounds useful in the methods are compounds that will cause crosslinking between the cellulose fibers. In one embodiment, the polymeric anionic reactive compounds are made of monomeric units having two carboxylic acid groups on adjacent atoms so that the carboxylic acid groups are capable of forming cyclic anhydrides which, at elevated temperature or other initiating force, will form an ester bond with the hydroxyl groups of the cellulose. Polymers, including copolymers, terpolymers, block copolymers, and homopolymers, of maleic acid are especially desired.
The present invention also is directed to high wet performance webs produced according to the methods of the invention and to articles made with the webs.
Webs are provided which exhibit high wet strength in one direction such as the machine or cross-machine direction, but which readily fail when wet in the orthogonal direction, providing easily flushable webs that nevertheless have good wet strength. The present invention can be used to produce flushable wet wipes, sanitary napkins, dry or pre-moistened bath tissue, and other absorbent products that have good integrity in the machine direction, for example, to resist elongational deformation or, more generally, to resist failure in use. The flushable products, by virtue of having regions that have not been treated with wet strength agents and specifically with polymeric anionic reactive compounds, have regions that can break apart readily when flushed and sent to a septic system, yet still have wet strength zones to enhance use prior to flushing.